The present invention relates generally to a measurement method, and more particularly to a measurement method for measuring a distortion of a projection optical system used in an exposure apparatus for fabricating various devices including semiconductor chips such as ICs and LSIs, display devices such as liquid crystal panels, sensing devices such as magnetic heads, and image pick-up devices such as CCDs, as well as fine patterns used for micromechanics. The present invention is suitable for a measurement for distortion of a projection optical system of an exposure apparatus for exposing a pattern, for example, with line width of 100 nm and less.
Conventionally, during manufacturing, photolithography technology, a exposure apparatus using a projection optical system to project a circuit pattern formed on a reticle (mask) onto a wafer, etc., has been employed for transferring the circuit pattern of fine semiconductor devices such as semiconductor memory and logic circuit.
The minimum critical dimension transferred by the projection exposure apparatus or resolution is proportional to the wavelength of light used for exposure and inversely proportional to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, or the higher the NA is, the better the resolution. Thus, along with recent demands for finer semiconductor devices, shorter ultraviolet light wavelengths have been proposed—from an ultra-high pressure mercury lamp (I-line with a wavelength of approximately 365 nm) to KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm). In recently, F2 laser (with a wavelength of approximately 157 nm) has been developed for a next light source.
On the other hand, high superposition accuracy has been required when it comes to satisfying the fine processing of a circuit pattern. There is a distortion (a distortion aberration) in a projection optical system of an exposure apparatus that deteriorates superposition accuracy, and it is necessary to suppress a distortion in a projection optical system so as to improve superposition accuracy along with a fine processing in the future.
In a recent lithography, a pattern with a line width of 100 nm or less is formed. The pattern of the nanometer order may have different problems in a conventional definition (see, “Offside” of “Oplus E”, published by New Technological Communications Company, issued in March 2003). Especially, a distortion causes acute problems by changing a line width when superposition accuracy is improved.
In a definition of the distortion by Zaidel, a principal ray as a center of light has a crossing position on an image surface (i.e., a displacement of an entire point on the image). Therefore, the distortion does not change even if a line width of an image formation changes.
However, a projection optical system for projecting an image of a reticle pattern has little coma aberration because of a design value and a manufacturing error. In other words, the image that is imaged by the projection optical system distorts and becomes asymmetric because of the coma aberration, and shifts from an ideal position that is without the coma aberration. The amount shifted from the ideal position that is without the coma aberration is an actual distortion (i.e., not a definition of Zaidel). Therefore, when the line width of the pattern changes, an influence by the coma aberration is different depending on the line width because an angle of diffraction light is different. A position of the image is also different because a distortion of the image is different. Thus, an actual distortion is different for each line width.
When an NA of a projection optical system changes, the coma aberration also changes. In this time, if the line width changes, an influence by the coma aberration is different depending on the line width because an angle of diffraction light is different. A position of an image is also different because a distortion of the image is different. Thus, an actual distortion is different for each line width.
Further, the projection optical system has the actual distortion when it has a little spherical aberration of, for example, about λ/10, results in a poor uniformity of an illumination optical system that illuminates a reticle pattern. When an exposure apparatus has various illuminating modes, the distortion is different for each of line widths in each of the illuminating modes.
Thus, a distortion is generated, and superposition accuracy is deteriorated when a line width of the formed circuit pattern changes.
However, a conventional superposition inspecting apparatus was not able to measure a distortion in the line width because a measurement principle of the conventional superposition inspecting apparatus adapts a characteristic of an image formation in an imaging optical system of a superposition inspecting apparatus and especially a method as “a bright field illumination+image data processing”.
For example, when a numerical aperture (NA) is 0.9 or more, and a wavelength of light used is 300 nm, a minimum size of the line width that can be separated as a video signal is about 0.8 μm for a line and space in an image formation of the imaging optical system of the superposition inspecting apparatus. A pattern with 100 nm or less which is imaged in recent lithography cannot be finely imaged in a contrast.
In other words, a method that can inspect a measurement of a distortion in a projection optical system of an exposure apparatus that uses the pattern of 100 nm or less with highly accurate and a high throughput does not currently exist.